1. Field of the Invention
The invention relates generally to a magnetic field sensing system that uses a current-perpendicular-to-the-plane (CPP) sensor like that used for giant magnetoresistive (GMR) and tunneling magnetoresistive (TMR) spin-valve (SV) sensors, and more particularly to a magnetic field sensing system that operates the CPP sensor in a mode different from conventional GMR-SV and TMR-SV systems.
2. Background of the Invention
Systems for sensing magnetic fields are well-known, including systems for reading of magnetically recorded data from disks in magnetic recording disk drives. One type of disk drive data-reading system uses a conventional magnetoresistive (MR) “spin-valve” (SV) sensor or read head. A SV MR sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu). One ferromagnetic layer has its direction of magnetization fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and the other ferromagnetic layer has its direction of magnetization “free” to rotate in the presence of an external magnetic field. With a sense or bias direct current applied to the sensor, the rotation of the free-layer magnetization relative to the fixed-layer magnetization is detectable as a change in electrical resistance.
In a magnetic recording disk drive SV read sensor or head, the stack of layers are located in the read “gap” between magnetic shields. The magnetization of the fixed or pinned layer is generally perpendicular to the plane of the disk, and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field. When exposed to an external magnetic field from the recorded data on the disk, the free-layer magnetization will rotate, causing a change in electrical resistance. A current-perpendicular-to-the-plane (CPP) SV sensor operates with the sense or bias direct current directed perpendicular to the planes of the layers in the sensor stack. CPP giant magnetoresistive SV read heads are described by A. Tanaka et al., “Spin-valve heads in the current-perpendicular-to-plane mode for ultrahigh-density recording”, IEEE TRANSACTIONS ON MAGNETICS, 38 (1): 84-88 Part 1 January 2002.
Another type of CPP sensor is a magnetic tunnel junction (MTJ) sensor, also called a tunneling MR or TMR sensor, in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer. In a CPP-TMR sensor the tunneling current perpendicularly through the layers depends on the relative orientation of the magnetizations of the two ferromagnetic layers. While in a CPP-SV read head the spacer layer is formed of an electrically conductive material, such as Cu, in a CPP-TMR read head the spacer layer is formed of an electrically insulating material, such as TiO2, MgO or Al2O3.
In magnetic field sensing systems with CPP sensors, it is desirable to operate the sensors at a high bias current density to maximize the signal and signal-to-noise ratio (SNR). However, it is known that CPP sensors are susceptible to current-induced noise and instability. The spin-polarized bias current flows perpendicularly through the ferromagnetic layers and produces a spin-torque effect on the local magnetization. This can produce continuous gyrations or excitations of the magnetization, resulting in substantial low-frequency magnetic noise if the bias current is greater than a “critical current” (IC). This effect is described by J.-G. Zhu et al., “Spin transfer induced noise in CPP read heads,” IEEE Transactions on Magnetics, Vol. 40, January 2004, pp. 182-188. Thus the adverse effect of spin-torque limits the bias current at which the CPP sensors can operate.
What is needed is a magnetic field sensing system, such as the data-reading system in a magnetic recording disk drive, that uses a CPP sensor but that operates in the presence of current-induced spin-torque without adverse effects.